There are two natural isotopes of lithium (Li) in nature, i.e., 7Li and 6Li, which make up 92.48% and 7.52% respectively. The nuclear reaction properties of the two isotopes are quite different, although both two isotopes play important roles in nuclear material field. 7Li is an essential molten salt coolant in thorium molten salt reactors. Since the thermal neutron absorption cross section of 6Li is quite high, which is 941 barns, while that of 7Li is only 0.033 barns, the molten salt reactor requires that the abundance of 7Li is >99.995%. Meanwhile, 7Li of high purity is used to adjust the pH value of primary coolant in pressurized-water reactors and is also used as a heat-conducting agent in fusion reactors. 6Li is a fuel in nuclear fusion reactors, wherein the abundance of 6Li should be >30%. In thorium molten salt reactors or in fusion reactors, lithium isotopes are used as indispensable strategy materials and energy materials, and provide a solution for the development of new energy.
The lithium isotope separation methods reported in the literature comprise: physical methods (such as an electromagnetic method, molecular distillation method and gas diffusion method, etc.) and chemical methods (such as electro-migration, electrolysis, a lithium-amalgam exchange method and solvent extracting exchange method, etc.) (Xiaoan Xiao et al, Journal of Nuclear and Radiochemistry, 1991, 13, 1). In the isotope separation field, physical methods are advantageous for heavy isotopes; while for light isotopes, the chemical methods are of higher efficiency and the physical methods are of low efficiency and great investment.
Since the lithium isotopes are light isotopes, and there is no lithium gaseous state, the lithium isotope separation by physical methods is only in the exploratory stage. With respect to chemical methods, most research in the field only involves determination and improvement of single stage isotope separation factor in the laboratory instead of multiple-stage separation. Meanwhile, in the chemical methods, gas-liquid chemical exchange method is not applicable for lithium, while the solid-liquid chemical exchange method is difficult to achieve a countercurrent multi-stage cascade. Therefore, only a liquid-liquid chemical exchange method can be adopted. When using the solid-liquid chemical exchange method, the crown ether polymers described in applications e.g., CN201210274233.1 and CN201210274356.5, possess a relatively high separation factor, but it is difficult to achieve a countercurrent multi-stage cascade with the solid-liquid chemical exchange method. When the liquid-liquid chemical exchange method is used, the single stage isotope separation factor α of the Sudan I-TOPO system is usually about 1.010 (Chen et al., Atomic Energy Science and Technology, 1987, 21, 433). However, a countercurrent multi-stage cascade has not been reported.
In order to be a practical lithium isotope separation method, it requires not only a large single stage isotope separation factor α of the separation system, but also a multi-stage cascade to be achieved in the chemical process. Thus a special cascade process, in which there is a strict method of controlling reflux, is required. The reflux must be thorough, easy to use and of low energy consumption. A precise feeding and reclaiming process must be achieved. The flow rate and flowing situation in the isotope enriching segment should be calculated and controlled so that a multi-stage cascade form is used to cumulatively enrich lithium-7 isotope products in high abundance.
So far, the multi-stage cascade enrichment process has not been reported in any of the above-mentioned physical or chemical methods. Only the lithium amalgam chemical exchange method has relatively good chemical properties and chemical technology, and has become the only industrial method for producing lithium-7 isotope (Chemical isotope separation principle, edited by Qiu Ling, Atomic Energy Press, 1990, pp 156-181). However, this process requires a lot of mercury which easily becomes volatile and runs off, is harmful to the operators' health and causes serious environmental pollution. Moreover, in this process, the sodium amalgam phase easily enriches 6Li. As lithium amalgam in the extraction column gradually decomposes, the method is difficult to meet the 7Li abundance requirement for thorium-based molten reactors, which is greater than 99.995%. Based on the characteristics and phase inversion requirements of lithium enrichment and separation, it is beneficial to choose organic phases which easily extract lithium and enrich for 7Li.